Renal Physiology Step 1: Glomerular Filtration - Video Tutorials & Practice Problems
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1
concept
The Filtration Membane
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4m
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In this video, we're gonna be talking about the filtration membrane. Now, if you have ever had a urine test done at a doctor's office, you might remember that they tested your urine or trace amounts of blood or excessive protein. And that's because the presence of, of blood or proteins in your urine can indicate a problem with your kidneys. So your kidneys have to find a way to filter out this huge volume of fluid from your blood while holding back all of your blood cells and platelets and most proteins. And that is where our filtration membrane comes in. So the filtration membrane is just the membrane between the capillaries and the capsular space. And it's gonna allow the passage of water and any solus smaller than plasma proteins. And the filtration membrane has three layers and you actually already know about two of these. So it's gonna be a nice easy lesson for you. So first we have the penetrated um endothelium of the glomerular capillaries. So these are just the big old cores in those leaky capillaries that, that we've, that we've talked about before. So as we've mentioned, those penetrations are gonna allow blood components except blood cells and platelets to pass through. And the gaps of these pores are quite large. They're about 70 to 100 nanometers. So again, pretty much everything except blood cells and platelets would be able to get through these gaps. Next, we have our basal lamina. So the basal lamina is a thin layer of extracellular matrix gel between the two other layers. And this gel actually has a negative charge which helps it to repel negatively charged plasma proteins like albumin and globulin. And the gaps in this layer are quite tiny. They're about eight nanometers. You can see there's a big jump there between the big pores of those capillaries and the gaps in the basal lamina. And then finally, we have our um our finest layer of the filtration membrane. We have the filtration slits of those poochy. So remember, the potto sites have those foot processes or pedestals that wrap around the glomerular capillaries and they interlace to form the filtration lips. And again, this is the finest layer and the gaps here are gonna be approximately 6 to 7 nanometers. And so the filtrate that will end up in the capsular space is going to contain water. It'll contain ions like sodium, potassium chloride, um nutrients like glucose and amino acids as well as waste products like urea and uric acid. If we come down to our image here, you can see what we are essentially looking at is a zoom in of um this wall of the capillary here where filtrate is gonna be coming out. And so we can see here on the left side of our zoom in what we're looking at here is the inside of the capillary. So we have like a red blood cell, a white blood cell, we've got platelet, big old plasma proteins and then teeny tiny solute like ions all kind of floating around in there. And then on the right side of the image, this tan area is our capsular space where our filtrate is going to end up. And then in the center, we have our filtration memory in there. They can see we have the unrated endothelium of the capillaries here. You see they have those kind of or fairly large pores in between them. There's plenty of space for things like um like the plasma proteins to get through that layer. But then we have this kind of mint green um basal lamina in between. Again, this layer is quite fine about eight nanometer gap. And it's gonna have that negative charge that will help repel those plasma proteins. And then we have our finest layer with our filtration splits. And you can see how teeny tiny those um openings would be and really only the tiniest valued ions, water molecules, et cetera would be able to get through there. All right. So that is our filtration membrane and I'll see you guys in the next one. Bye bye.
2
example
Renal Physiology Step 1: Glomerular Filtration Example 1
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42s
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OK. So which of the following would not be able to pass through the fenestrated endothelium of the glomerular capillary. So keep in mind that this is going to be the layer of our filtration membrane that has the largest gap. And the point of this layer is to allow all blood components except for blood cells and platelets to get through. So our answer here is going to be the red blood cells. We don't want any blood cells ending up in our urine. They need to stay in our capillaries, right? And keep in mind that ions, water molecules and glucose are all tiny enough to end up in our filtrate. So all of those substances are small enough to get through all three layers of the filtration membrane. So our answer here is feet, the red blood cells.
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Problem
Problem
The ________________ is the finest layer of the filtration membrane.
A
Fenestrated endothelium of the glomerular capillaries.
B
Basal lamina.
C
Filtration slits of the podocytes.
4
concept
Overview of Filtration Pressures
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2m
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OK. So now that we're getting into renal physiology, we're gonna be talking quite a bit about glomerular filtration pressure as well as glomerular filtration rate. But to understand though, we're first gonna just do a quick overview of filtration pressures and how they work in our body. So, as you may recall from, from some earlier chapters, there are two main forces that drive fluid movement in a capillary bed. So the first of these is hydrostatic pressure and hydrostatic pressure is the force of a fluid on the wall of its container or basically on the like wall of that capillary. And this is usually about equal to blood pressure. So what's happening here is that the pressure of the blood is literally just pushing the blood up against the wall of the capillary. And then if that capillary is leaky enough, um we're gonna see some water and other small solu potentially get pushed out of the capillary. So if you look at our little cartoon here that's labeled hydrostatic pressure, you can see we have this wave of blood pushing against the wall of that capillary, this green wall. And then we have all of these little solus like sodium and potassium, as well as water molecules ending up in this kind of tan colored interstitial space. So that is broadly how hydrostatic pressure works. Now, our next force is colloid osmotic pressure. So colloid osmotic pressure is basically the pressure created by proteins mainly albumin that are in the plasma. So what happens here is that the proteins create an osmotic gradient that pulls water back into the capillary. So remember, water tends to move from low solu concentrations toward high solute concentrations. They're attracted to those high concentrations of plasma protein. Remember how we talked about a long time ago how like I always think of ions like sodium and potassium as like introverts like they want to follow that concentration gradient and get out of a crowd. Water is the opposite. Water is an extrovert. It wants to come join every party that it eats. So it's gonna be moving from low solu concentrations toward high salute concentration. So it's attracted to all those proteins in our capillary. If you look at our little cartoon over here labeled colloid osmotic pressure, you can see in the capillary, we have a little plasma protein party, there's plenty of plasma hanging out and the water molecules over here are like, hey, that looks super fun. We wanna go over there and so they get drawn back into that capillary to join that little party. So that is broadly how colloid osmotic pressure work. Now, when you combine these, you end up with net filtration pressure. And so net filtration pressure will determine the direction of fluid movement as well as the force um between the capillaries and the interstitial fluid. All right. So that is our quick overview. And after a quick example, I will see you guys in our next video to talk about how this actually works in our kidneys. So I'll see you there.
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example
Renal Physiology Step 1: Glomerular Filtration Example 2
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32s
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All right. So water moves out of the capillary if blank is higher than blank. Remember between our two forces here, hydrostatic pressure and colloid osmotic pressure. Hydrostatic pressure is the one that will be forcing fluids out of the capillary because basically the blood pressure is pushing fluid up against the wall of that capillary. And so fluid can potentially escape if the capillaries are gonna be leaky enough. And so our answer here is going to be water moved out of the capillary if hydrostatic pressure is higher than colloid osmotic pressure, and there you go.
6
concept
Glomerular Filtration Pressure
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3m
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OK. So now that we understand filtration pressures more generally, let's talk about glomerular filtration pressure. So, glomerular filtration pressure is determined by three factor. So first up, we have glomerular hydrostatic pressure. And the main principle at work here is of course, hydrostatic pressure. So this one will be largely determined by systemic blood pressure. And so what's happening here is high resistance causes the blood to push against the walls of the glomerular capillaries. And so this force favors filtration. Basically, the um blood pressure will be pushing fluid through the filtration membrane. So the actual pressure happening is pushing fluid against that membrane. And so you can see in our image, we have our blood, it's pushing against that filtration membrane. And then all of the water molecules electrolyte, you know, little tiny solus and get through and end up in the filtrate. And this is going to be a pretty powerful force, gonna be about 50 to 55 millimeters of mercury. Now, our next force is capsular hydrostatic pressure. And so this is also just the the basic principle of hydrostatic pressure. But what's happening here is that the filtrate in the capsular space builds up its own hydrostatic pressure. And so this force is actually gonna be opposing filtration because what's happening is that the hydrostatic pressure of that filtrate pushes fluid back into the capillary or it's pushing against the filtration membrane from the opposite side. You can see we have this filtrate here and the hydrostatic pressure of that filtrate is pushing on the filtration membrane in the opposite direction. And this horse is lucky for us pretty weak. It's about 10 to 15 millimeters of mercury. And then our final pressure is glomerular colloid osmotic pressure. The main principle here, of course, being colloid osmotic pressure. And so what's happening is that the high concentration of asthma proteins um specifically albumin within the capillaries is creating an osmotic gradient. Remember, water wants to move from low salute concentration or high solu concentration and because the blood in the capillaries is now highly concentrated since all the fluid has moved out that is looking very attractive for all those water molecules. And so this force also opposes filtration. So the osmotic gradient is drawing water back into the capillaries. That's where the water at least wants to be. They can see in our, in the little cartoon that we just saw in the previous video, we have our little water molecules hanging out in the filtrate and they're seeing that high concentration of plasma proteins and they're like, hey, we wanna go join that party, right? So they're getting drawn back toward the capillary and this course is gonna be about 30 millimeters of mercury. Now when you add all those numbers up together, our net glomerular filtration pressure is going to be about 10 millimeters of mercury favoring movement through the filtration membrane. This number is actually very important. Our body really wants to keep that pretty consistent because if this if this number changes by even like 10 to 18% it can actually have devastating effects on the body, which we will talk more about in our glomerular filtration rate video. So I will see you guys in our example that we can learn how to actually solve for this number together. So I will see you guys there.
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example
Renal Physiology Step 1: Glomerular Filtration Example 3
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43s
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OK. So we are gonna solve or net filtration pressure together using this equation here. So based on this equation, net filtration pressure is going to be equal to glomerular hydrostatic pressure. So we'll call that 50 minus and then in parentheses, capsular hydrostatic pressure. So we'll just use 10 plus glomerular colloid osmotic pressure which is 30. So 10 plus 30 is 40 then 50 minus 40 of course is 10. So that is how we get that 10 millimeters of mercury. And again, we will talk about why that number is so important in our video on glomerular filtration rate. So I'll see you there.
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Problem
Problem
In the process of filtrate formation, which of the following factors creates an osmotic gradient?
A
Systemic blood pressure.
B
A high concentration of negative ions in the capillaries.
C
A high concentration of plasma proteins in the capillaries.
D
A low concentration of water in the capsular space.
9
concept
Glomerular Filtration Rate
Video duration:
3m
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In this video, we're gonna be talking about glomerular filtration rate. Now, glomerular filtration pressure, which we just talked about previously directly impacts glomerular filtration rate. And glomerular filtration rate is just the amount of filtrate formed by both kidneys in one minute. It's on average, it's gonna be approximately 100 and 25 mL per minute. Now, on average, what we see is that in healthy individuals, blood pressure and glomerular filtration rate are positively correlated. So generally, what's going on here is that systemic blood pressure is the strongest predictor of glom predictor of glomerular filtration pressure. And then glomerular filtration pressure is the strongest predictor of glomerular filtration rate. And so what we end up with is that if systemic blood pressure increases, typically in a healthy person, glomerular filtration rate will also increase if systemic blood pressure decreases, glomerular filtration rate will also decrease. So that those variables are always moving in the same directions. We have a beautiful positive correlation there. And again, this is on average in healthy individuals that this is the pattern that we would expect to see. Now, glomerular filtration rate is highly regulated by the body. And that is because it has a very strong impact on blood volume, blood pressure as well as just our general homeostasis and health. So just to give you some examples of what can happen when glomerular filtration rate kind of gets out of whack. If we were to have chronically increased glomerular filtration rate, we would end up with an increased urine output because we're just making more filtrate and more filtrate means more urine. Because of that increased urine output, we would have a decreased blood volume because water is leaving the body, right. We have decreased blood volume, which would lead to decreased blood pressure, that can lead to things like dehydration as well as electrolyte imbalances. We're gonna be losing a lot more electrolytes um in all of that urine. Now, on the flip side, if we were to have chronically decreased glomerular filtration, we're gonna end up with a decreased urine output where it's not gonna urinate. As often, we're gonna be retaining all of that water in our body, which will lead to increased blood volume as well as increased blood pressure. And then the adverse of the effects of that could be things like hypertension edema or swelling as well as the retention of waste products and toxins within our body because they're obviously not getting excreted in urine um at the correct rate. So you can see why glomerular filtration rate really needs to be kind of held at a nice constant and we're going to talk. We're going to spend the next couple of videos talking about many of the ways that our body monitors and regulates glomerular filtration rate. So I'll see you there.
10
example
Renal Physiology Step 1: Glomerular Filtration Example 4
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51s
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Ok. So Caitlyn is a 25 year old woman. She has no underlying health conditions and does not take any medication when her blood pressure increases. What outcome would you expect to see? So, remember, blood pressure, glomerular filtration pressure and glomerular filtration rate are all going to be positively correlated. They're all gonna be moving in the same direction. So if blood pressure increases, we're gonna expect glomerular filtration pressure to also increase. So based on that, we can narrow it down to either B or C. And then if glomerular filtration pressure increases, we're gonna expect glomerular filtration rate to increase. So it looks like our answer is B as blood pressure increases, our glomerular filtration pressure increases and then our glomerular filtration rate increases as well. So there you go.
11
Problem
Problem
Which of the following is a possible consequence of a prolonged or chronic decrease in glomerular filtration rate?
A
Dehydration.
B
Leukemia.
C
Edema (swelling).
D
Hypotension.
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